There are a few camping spots on origins where you can get 50 + legit. The best is at Juggernog using the Icestaff so when the panzers come you can train the first hoard of zombies while you kill them.

If you camp 2 player it would be better if the second player had the fire staff because it kills the panzers very easy. All you need to do is shoot 3 burst shots at the floor in front of you with the Icestaff and 6 with the Firestaff (the Firestaff is only effective camping with a 6 burst otherwise it does not last long enough).

Another good camping spot is semtex (near the the door that is between spawn room and generator 3), but if you go down here its alot harder to get back into the game. The spawns come a lot faster if you leave the spawn door and the door in front of you closed, you just have to jump down here from the tank road and to leave this area with out opening the doors you just go to the crazy place via the fire tunnel.

These camping strategies will eat through ammo very quickly so at some points in the game when your ammo is low, you will have to train.

It is better not to let the random box move from it's starting position, if it doesn't move you will not get firesales and you will get more ammo drops and instakills throughout the game.